1. Field of the Invention
This invention relates to the separation of mineral matter (ash) from coal. It specifically relates to the use of liquid carbon dioxide for removing mineral matter from coal.
2. Description of the Prior Art
U.S. Pat. No. 2,142,207 discloses a froth flotation process for cleaning coal. Gaseous carbon dioxide is bubbled upwardly through an aqueous slurry of coal to form a froth floating on top of the slurry. The froth is then removed. There is no specific mention of deashing coal by the method. This method requires a pressure release to allow escape of the carbon dioxide gas and thereby accomplish frothing. It is apparent that pressure release is incompatable with the use of liquid carbon dioxide.
U.S. Pat. No. 3,998,604 discloses the use of carbon dioxide gas as a coal flotation agent. However, this patent fails to disclose the use of liquid carbon dioxide.
U.S. Pat. No. 4,198,291 discloses the separation of coal from minerals using liquid sulfur dioxide. However, liquid sulfur dioxide is not equivalent to liquid carbon dioxide because sulfur dioxide is an extremely noxious gas, while carbon dioxide is entirely harmless. In addition, unlike sulfur dioxide, carbon dioxide is readily available in very large quantities through onsite combustion of coal.
U.S. Pat. No. 4,206,610 discloses a method for transporting coal by slurrying finely divided coal in liquid carbon dioxide. However, this patent does not disclose the use of a liquid carbon dioxide phase in cooperation with a water phase for the separation of mineral matter from coal.
In spite of the prior art teachings, there remains a need for a non-toxic, low cost material which will accomplish the separation of minerals from coal in a simple manner.